You have to adjust the necessary settings in advance so that the system can make all necessary changes before the promotion starts. If you create a promotion and adjust all settings in advance, the systems will calculate the necessary inventory levels and you will be prepared for the promotion.
The system has several options that can be used to change current inventory levels. They can be changed according to the coefficient of current sales, or average sales, or sales within a certain period. Another option is to set a special quantity value which will be kept during promotion period, or find the quantity value on the basis of similar past promotion sales.
As soon as the promotion is over, the system will return back to the normal pre-promotion inventory levels.